The present invention relates to a new antenna design. The antenna is directional and is preferably of a thin, flat construction. The antenna has multiple elements which provide directivity. The antenna may be flush-mounted on a high impedance surface. The antenna may be used with beam diversity hardware, for example, to improve the signal transmission and reception of wireless communications. Since the antenna may be flush-mounted, it can advantageously used on a mobile platform such as an automobile, a truck, a ship, a train or an aircraft.
Prior art antennas and technology includes:
T. Schwengler, P. Perini, xe2x80x9cCombined Space and Polarization Diversity Antennasxe2x80x9d, U.S. Pat. No. 5,923,303, Jul. 13, 1999. An antenna system with both spatial and polarization diversity has a first antenna aperture and a second antenna aperture, with a polarization separation angle being formed by the difference between the polarization angle of the first antenna aperture and the polarization angle of the second antenna aperture, and a vertical separation being formed by mounting the second antenna aperture a vertical distance above the first antenna aperture, such that diversity gain is achieved by both the polarization angle and the vertical distance. The combination of spatial and polarization diversity allows closer antenna aperture spacing and non-orthogonal polarization angles. However, using current techniques, antennas having both polarizations can not lie in a single planexe2x80x94so the resulting antenna is not a low-profile antenna like the antenna disclosed herein.
M. Schnetzer, xe2x80x9cTapered Notch Antenna Using Coplanar Waveguidexe2x80x9d U.S. Pat. No. 5,519,408. Tapered notch antennas, which are sometime known as Vivaldi antennas, may be made using standard printed circuit technologies.
D. Sievenpiper, E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d U.S. Provisional patent application, serial number 60/079953, filed on Mar. 30, 1998.
It is also known it the prior art to place a conformable end-fire antenna or array on a Hi-Z surface. It has been shown that the Hi-Z material can allow flush-mounted antennas to radiate in end-fire mode, with the radiation exiting the surface at a small angle with respect to the horizon.
Conventional vehicular antennas consist of a vertical monopole which protrudes from the metallic exterior of vehicle, or a dipole embedded in the windshield or other window. Both antennas are designed to have an omnidirectional radiation pattern so signals from all directions can be received. One disadvantage of omnidirectional antennas is that they are particularly susceptible to interference and fading, caused by either unwanted signals from sources other than the desired base station, or by signals reflected from vehicle body and other objects in the environment in a phenomenon known as multipath. Antenna diversity, in which several antennas are used with a single receiver, can be used to help overcome multipath problems. The receiver utilizing antenna diversity switches between the antennas to find the strongest signal. In more complicated schemes, the receiver can select a linear combination of the signals from all antennas.
The disadvantage of antenna diversity is the need for multiple antennas, which can lead to an unsightly vehicle with poor aerodynamics. Many geometries have been proposed which reduce the profile of the antenna, including patch antennas, planar inverted F-antennas, slot antennas, and others. Patch and slot antennas are described by, C Balanis, Antenna Theory, Analysis and Design, 2nd ed., John Wiley and Sons, New York (1997). Planar inverted F-antennas are described by M. A. Jensen and Y. Rahmat-Samii, xe2x80x9cPerformance analysis of antennas for handheld transceivers using FDTD,xe2x80x9d IEEE Trans. Antennas Propagat., vol. 42, pp. 1106-1113, August 1994. These antennas all tend to suffer from unwanted surface wave excitation and the need for thick substrates or cavities.
As such, there is a need for an antenna which has low profile and has sufficient directivity to take advantage of antenna diversity. Preferably the antenna should not suffer from the effects of surface waves on the metal exterior of the vehicle.
The high impedance (Hi-Z) surface,which is the subject of U.S. Ser. No. 60/079953 mentioned above, provides a means of fabricating very thin antennas, which can be mounted directly adjacent to a conductive surface without being shorted out. Near the resonance frequency, the structure exhibits high electromagnetic impedance. This means that it can accommodate non-zero tangential electric fields at the surface of a low-profile antenna, and can be used as a shielding layer between the metal exterior of a vehicle and the antenna. The totals height is typically a small fraction of a wavelength, making this technology particularly attractive for mobile communications, where size and aerodynamics are important. Another property of this Hi-Z material is that it is capable of suppressing the propagation of surface waves. Surface waves normally exist on any metal surface, including the exterior metal skin of a vehicle, and can be a source of interference in many antenna situations. Surrounding the antenna with a small area of Hi-Z surface can shield the antenna from these surface waves. This has been shown to reduce multipath interference caused by scattering from ground plane edges.
The present application is related to (i) U.S. patent application Ser. No. 09/520,503 entitled xe2x80x9cA Polarization Converting Radio Frequency Reflecting Surfacexe2x80x9d filed Mar. 8, 2000, and to (ii) U.S. patent application Ser. No. 09/525,831 entitled xe2x80x9cPlanar Antenna with Switched Beam Diversity for Interference Reduction in a Mobile Environmentxe2x80x9d filed Mar. 15,2000, the disclosures of which are hereby incorporated herein by this reference.
The Hi-Z surface, which is the subject matter of U.S. patent application Ser. No. 60/079,953 and which is depicted in FIG. 1a, includes an array of resonant metal elements 12 arranged above a flat metal ground plane 14. The size of each element is much less than the operating wavelength. The overall thickness of the structure is also much less than the operating wavelength. The presence of the resonant elements has the effect of changing the boundary condition at the surface, so that it appears as an artificial magnetic conductor, rather than an electric conductor. It has this property over a bandwidth ranging from a few percent to nearly an octave, depending on the thickness of the structure with respect to the operating wavelength. It is somewhat similar to a corrugated metal surface 22 (see FIG. 1b), which has been known to use a resonant structure to transform a short circuit into an open circuit. Quarter wavelength slots 24 of a corrugated surface 22 are replaced with lumped circuit elements in the Hi-Z surface, resulting in a much thinner structure, as is shown in FIG. 1a. The Hi-Z surface can be made in various forms, including a multi-layer structure with overlapping capacitor plates. Preferably the Hi-Z structure is formed on a printed circuit board (not shown in FIG. 1a) with the elements 12 formed on one major surface thereof and the ground plane 14 formed on the other major surface thereof. Capacitive loading allows a frequency be lowered for a given thickness. Operating frequencies ranging from hundreds of megahertz to tens of gigahertz have been demonstrated using a variety of geometries of Hi-Z surfaces.
It has been shown that antennas can be placed directly adjacent the Hi-Z surface and will not be shorted out due to the unusual surface impedance. This is based on the fact that the Hi-Z surface allows a non-zero tangential radio frequency electric field, a condition which is not permitted on an ordinary flat conductor.
In one aspect the present invention provides an antenna comprising a plurality of flared notch antennas disposed immediately adjacent each other. Each flared notch antenna has a direction of maximum gain which is directed in a different direction for each flared notch antenna and is defined by a pair of confronting elements, each confronting element being associated with two different ones of the plurality of flared notch antennas. Each confronting element has a gap therein having a length which is approximately equal to a quarter wave length of a radio frequency signal to be received and/or transmitted by the antenna.
In another aspect the present invention provides an antenna comprising a high impedance surface; and a plurality of Vivaldi flared notch antennas disposed immediately adjacent the high impedance surface. Each Vivaldi Flared notch antenna is formed by two generally planar conductive elements disposed in a confronting relationship, with a feed point being defined therebetween. Each Vivaldi flared notch antenna shares each of its two planar elements with a different adjacent Vivaldi flared notch antenna.
In yet another aspect the present invention provides an antenna for receiving and/or transmitting a radio frequency wave, the antenna including a plurality of flared notch antennas disposed adjacent to each other and arranged such that their directions of maximum gain point in different directions. Each of the flared notch antennas is associated with a pair of radio frequency radiating elements and each radio frequency radiating element (i) serves as a radio frequency radiating element for two different flared notch antennas and (ii) has a gap therein having a length equal to approximately one quarter wavelength of the radio frequency wave.